Joseph Cap was a gun and barrel maker
http://www.littlegun.be/arme%20belge/artisans%20identifies%20c/a%20cap%20joseph%20gb.htmI don't know from whom he sources his steel, but likely
Acier CockerillJean Falla used his steel, and also Poldi. He was registered 1931-1953
http://www.littlegun.be/arme%20belge/artisans%20identifies%20e%20f/a%20falla%20gb.htmSo it was Nickel Steel, likely AISI 23XX. I have no information regarding actual composition however.
Krupp Nickel Steel was patented in 1890 and had a reported tensile strength of 92,500 psi. The composition of Krupp gun barrel nickel steel may have changed over the years.
The pre-1920 Winchester Nickel Steel specimen I tested was non-standard AISI 2340 (just slightly low Ni and Mn) with a tensile strength of 124,000 psi. Impressive stuff.
In general, like Damascus, the quality of the barrel steel is reflected by the overall quality of the gun. High $/Franc guns were fitted with high $/Franc steel barrels.